The present invention relates generally to developing devices which are adaptable for use in image forming apparatuses, typically including printers and copying machines.
Currently available image forming apparatuses for the formation of images by use of known electrophotography processes are typically designed to form a toner image on a photosensitive body serving as an image carrier body by a method including the steps of electrostatically charging the photosensitive body up to a specified voltage potential, then performing image exposure to form an electrostatic latent image on the photosensitive body, and supplying developing powder onto a surface of the photosensitive body on which this electrostatic latent image is held to thereby develop the electrostatic latent image. The toner image thus formed on the photosensitive body is transferred to a recording material (paper sheet in most cases) and then outwardly transferred as a printed sheet from the image forming apparatus.
A developing device such as widely used with an image forming apparatus of the type described above is generally designed to include a development unit and a toner tank unit. Here, the development unit is a component which includes a developing powder storage unit for storage of developing powder, a development roller for feed and delivery of the developing powder within said developing powder storage unit toward the photosensitive body, and a toner density sensor for detection of the density of the toner of such developing powder within said developing powder storage unit. In addition, the toner tank unit is a component that includes a toner storage unit for storage of toner, along with toner delivery means for delivery of toner out of said toner storage unit.
In the developing device of the type described above, the toner in the developing powder within the developing powder storage unit is gradually consumed. In cases where the toner density in the developing powder decreases and becomes lower than a preset toner density reference value, the toner density sensor operates to detect this state and then to control the toner delivery means to additionally supply toner within the toner tank unit to the developing powder storage unit.
Unfortunately, the prior art image forming apparatus has a problem in that, in the case of an arrangement for directly supplying toner from the toner tank unit to the development unit in the way discussed above, a supply of toner to the surface of the photosensitive body is unintentionally carried out at a time when the developing powder within the development unit and the toner that has been newly supplied to the development unit are not yet sufficiently stirred to provide a proper mixture, which could result in formation of a toner image with density irregularities and/or production of printed matter with toner contamination at the background of a sheet of paper. This problem will readily take place especially when an attempt is made to speed up the developing device, and thus is one of the most important problems to be solved in the attempt to provide a developing device capable of providing for high-speed operations.
One prior known approach, which has been previously proposed by the present inventors, to solve the above-noted problem is to provide a specific developing device which is arranged to include a mixture/stirring chamber disposed between the toner tank unit and the development unit for mixture of the toner that is delivered out of the toner tank unit with developing powder in the stirring chamber prior to supplying this additional toner to the development unit, to thereby ensure that the development unit is filled only with toner that has a stabilized friction electrification state or the like (for detail see JP-A-10-177293).
The present invention is directed to an improvement of the developing device as disclosed in JP-A-10-177293, and its primary objective is to achieve a developing device with increased reliability of developing powder and toner feed/transfer control at a toner tank unit and preliminary mixture stirring unit, as well as in the development unit thereof. Another object of the invention is to provide a developing device which is capable of producing stable images even when continuous printing is carried out for an extended length of time.
The foregoing objects may be attained by providing a new and improved developing device for use in an image forming apparatus for supplying developing powder to an image carrier body having an electrostatic latent image on the surface thereof and for forming a toner image on said image carrier body. The developing device comprises a development unit including a developing powder storage unit storing therein developing powder and a developing powder supply means for supplying and delivering developing powder within said developing powder storage unit toward said image carrier body, as well as first toner density detector means for detecting the toner density of the developing powder within said developing powder storage unit. The developing device further includes a toner tank unit including a toner storage unit storing toner therein, and toner delivery means for outwardly delivering toner from said toner storage unit.
The developing device also includes a preliminary mixture/stirring unit including a stirring chamber for mixing the developing powder which is released from said developing powder supply means with the toner delivered from said toner tank unit, developing powder delivery means for delivering the developing powder out of said stirring chamber, second toner density detector means for detecting the toner density of the developing powder within said stirring chamber, developing powder amount detector means for detecting an amount of developing powder within said stirring chamber and regulator means for regulating the delivery of the developing powder that is released from said developing powder supply means into said stirring chamber.
A control means is operatively responsive to receipt of an output signal of said first toner density detector means for driving said developing powder delivery means to supply developing powder from said stirring chamber to said development unit, while controlling said regulator means to inhibit introduction of the developing powder into said stirring chamber at least within a specified time period for supplying said developing powder to said development unit.
In accordance with the present invention, it is possible to provide a developing device for use in an image forming apparatus which has improved reliability in the use of developing powder and in toner feed/transfer control at its toner tank unit and preliminary mixture stirring unit, as well as in the development unit thereof. It is also possible to achieve a developing device for an image forming apparatus which is capable of producing stable images even where continuous printing is carried out for an extended length of time.